Method for removing a contamination layer from an optical surface and arrangement therefor as well as a method for generating a cleaning gas and arrangement therefor

ABSTRACT

The invention is directed to a method for at least partially removing a contamination layer ( 15 ) from an optical surface ( 14   a ) of an EUV-reflective optical element ( 14 ) by bringing a cleaning gas into contact with the contamination layer. In the method, a jet ( 20 ) of cleaning gas is directed to the contamination layer ( 15 ) for removing material from the contamination layer ( 15 ). The contamination layer ( 15 ) is monitored for generating a signal indicative of the thickness of the contamination layer ( 15 ) and the jet ( 20 ) of cleaning gas is controlled by moving the jet ( 20 ) of cleaning gas relative to the optical surface ( 14   a ) using this signal as a feedback signal. A cleaning arrangement ( 19  to  24 ) for carrying out the method is also disclosed. The invention also relates to a method for generating a jet ( 20 ) of cleaning gas and to a corresponding cleaning gas generation arrangement.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation application of international patentapplication PCT/EP 2007/009593, filed Nov. 6, 2007, designating theUnited States and the entire content thereof is incorporated herein byreference.

FIELD OF THE INVENTION

The invention relates to a method for at least partially removing acontamination layer from an optical surface of an EUV-reflective opticalelement by bringing a cleaning gas, preferably comprising atomichydrogen, into contact with the contamination layer. The invention alsorelates to a cleaning arrangement for carrying out the method. A methodis also disclosed for generating a jet of cleaning gas, preferablycomprising atomic hydrogen, to be directed to a contamination layer onan optical surface of an EUV-reflective optical element. In addition, acorresponding cleaning gas generation arrangement as well as anEUV-lithography system are presented.

BACKGROUND OF THE INVENTION

Lithography exposure systems for the EUV wavelength range (from about 5nm to about 20 nm) generally comprise an EUV light source, anillumination system for homogeneously illuminating a pattern arranged ona mask with light from the EUV source, and a projection system forimaging the pattern onto a photosensitive substrate (wafer). In thepresent application, the term “light” designates electromagneticradiation at wavelengths which are not restricted to the visible domain,i.e. the term “light” will be used also for radiation in the EUV or VUVwavelength range.

During the exposure process, a contamination layer containing mainlycarbon grows on the optical surfaces of the optical elements of theEUV-lithography system. The formation of the contamination layer istriggered by radiation-induced cracking of hydrocarbon molecules, thepresence of which cannot be avoided even though the compartments of theEUV-lithography system are operated under vacuum conditions. Thecontaminating material, in particular carbon, can be cleaned away bybringing the contamination layer on the optical surfaces into contactwith a cleaning gas, such as atomic hydrogen. Currently, it is foreseento equip each EUV-reflective element of a EUV lithography system withone hydrogen radical generator (HRG), such that cleaning can beperformed in-situ, i.e. without removal of the optical elements from theEUV-lithography system. In the present application, the term “atomichydrogen” is used for all types of activated hydrogen (H₂), i.e. notonly designating hydrogen in the form of hydrogen radicals H•, but alsohydrogen ions such as H⁺ or H₂ ⁺ or hydrogen H* in an excited (electron)state.

In general, the amount of material removed from the contamination layerby the cleaning gas cannot be precisely determined. Consequently, thecleaning time during which the cleaning gas should be brought intocontact with the contamination layer is only approximately known. Incase that the cleaning time is too short, part of the contaminationlayer will not be removed from the optical surface, causing an unwantedreflection loss even after the cleaning. Therefore, for ensuring thatthe entire contamination layer is removed by the cleaning, a cleaningtime may be chosen which is too long (so-called “overcleaning”) so that,especially close to the end of the cleaning process, part of thecleaning gas may come into contact with the optical surface. As theoptical surface in general also reacts with the cleaning gas, anirreversible contamination is caused on the optical surface in a lowamount per cleaning cycle. As only 1% reflection loss due toirreversible contamination is permitted over the lifetime of aEUV-reflective element, the lifetime of the optics of a EUV-lithographysystem is determined by the mean time between successive cleaningprocesses multiplied by the number of allowed cleaning cycles.

In the publication US 2003/0051739 A1, a device for removing carboncontaminations from an optical surface of a mirror element in a EUVlithography system is disclosed. In one example, the device comprisestwo cleaning gas generators, each for generating a jet of cleaning gaswhich is directed to the optical surface. In another example, a singlecleaning gas generator of cylindrical shape is used which is situatedaround the perimeter of the mirror. The cleaning gas is generated byactivation of a supply gas using accelerated electrons which areproduced by thermoemission from a heated filament.

WO 2004/104707 A2 discloses a method for in-situ cleaning of an opticalsurface of an optical element for EUV or soft X-ray radiation which isarranged in a vacuum vessel, the optical surface being contaminated withan inorganic substance introduced by a radiation source. In the method,at least one reagent which is substantially translucent or transparentto the radiation (such as molecular hydrogen) is added through a supplysystem of a vacuum vessel, depending on the prevailing conditions. Thereagent chemically reacts with the contaminants to remove them from theoptical surface. The reagent may be activated by irradiation with anactivation light source and may be generated in a pulsed manner. Themethod may also be electronically controlled, e.g. by taking thethickness of the contamination layer into account.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a method for at leastpartially removing a contamination layer from an optical surface of anEUV-reflective element, a method for generating a cleaning gas, as wellas a corresponding cleaning arrangement and a corresponding cleaning gasgeneration arrangement, all of which are suitable for reducing oravoiding damage to the optical surface of the EUV-reflective elementwhich may otherwise be caused by the cleaning process.

According to a first aspect, a method as described in the introductionis provided, the method comprising the steps of: directing a jet ofcleaning gas to the contamination layer for removing material from thecontamination layer, monitoring the contamination layer for generating asignal indicative of the thickness of the contamination layer, andcontrolling the jet of cleaning gas by moving the jet of cleaning gasrelative to the optical surface, using the signal indicative of thethickness of the contamination layer as a feedback signal.

In the method proposed above, by monitoring the thickness of thecontamination layer and using the thickness of the contamination layeras feedback information for controlling the jet of cleaning gas, theinformation about the actual thickness of the contamination layer isavailable during the entire cleaning process. In such a way, it ispossible to take appropriate measures to avoid damaging of the opticalsurface which may be caused when the cleaning gas is brought intocontact with the optical surface. In particular, in case that thethickness of the contamination layer falls below a given threshold, thecleaning can e.g. be slowed down by reducing the flow of cleaning gas tothe optical surface, or the choice of the cleaning gas or of thecleaning method may be controlled in dependence of the thickness of thecontamination layer, i.e. using more abrasive cleaning methods when thethickness of the contamination layer is comparatively large andresorting to less aggressive cleaning methods when the thickness of thecontamination layer is reduced during the cleaning. In such a way, thecleaning can be accelerated while still avoiding irreversiblecontaminations of the optical surface caused by the cleaning gas.

During the illumination process, the contamination layer may grow on theoptical surface with a thickness distribution showing a local variationdepending on the position on the optical surface, yet, as such athickness variation may be moderate in some cases, as a firstapproximation, a contamination layer with a homogeneous thicknessdistribution may be assumed. In this case, i.e. when the contaminationlayer on the optical surface is sufficiently homogeneous, monitoring ofthe thickness of the contamination layer in only one point on theoptical surface may be sufficient. However, in case that thecontamination layer has an inhomogeneous distribution over the opticalsurface, it is advisable to measure the thickness in a spatiallyresolved way, preferably by monitoring the thickness distribution of thecontamination layer at each point of the optical surface.

It is to be understood that in the present application, the term“contamination layer” does not necessarily refer to a continuous layerof material deposited on the optical surface, i.e. the term“contamination layer” also encompasses contamination distributions witha discontinuous structure, e.g. forming contaminating spots or areas onthe optical surface which are not connected to each other. Also, theterm “control” is used herein both for open-loop and closed-loopcontrol.

In a preferred variant, the movement of the jet of cleaning gas iscontrolled by displacing or by changing the direction of the jet ofcleaning gas relative to the optical surface. By performing atranslatory motion or by tilting/rotating the jet of cleaning gas, thelocation of the impact zone of the jet of cleaning gas on thecontamination layer can be controlled, thus allowing removing materialfrom the contamination layer in a dedicated, i.e. spatially resolvedmanner. Alternatively or in addition, it is possible to modify the shapeand size of the impact zone of the jet of cleaning gas on thecontamination layer, e.g. by changing the direction of the jet ofcleaning gas or by modifying the distance between a source of cleaninggas (e.g. a cleaning head) and the optical surface. In contrast to this,in the state of the art, only a single static cleaning head is used,which due to the characteristics of the jets of cleaning gas only allowsfor cleaning in an inhomogeneous way, such that an unequal amount ofmaterial is removed from different areas of the contamination layer.Therefore, even with a contamination layer having essentially constantthickness, cleaning is performed in an inhomogeneous manner, i.e. insome areas more material is removed as compared to others. Therefore, inthe state of the art, in order to remove the entire contamination layerfrom the optical element, an over-cleaning of the optical surface cannotbe avoided, as for entirely removing the contamination layer also inthose areas where less material is removed per time unit, those areaswhere more material is removed will already be free from contaminations,such that the cleaning gas is inevitably brought into contact with theoptical surface in those areas.

In a preferred further development, the jet of cleaning gas is displacedor tilted in a scanning manner relative to the optical surface. In thisway, the material of the contamination layer can be removed from theoptical surface in a systematic way. Preferably, the size of the impactzone of the jet of cleaning gas on the optical surface is relativelysmall, e.g. about 10% or 5% of the overall area of the optical surface,allowing one to remove the material from the optical surface in adedicated way, thus increasing the uniformity of the cleaning, and, inparticular, adapting the amount of locally removed material independence of the local thickness of the contamination layer.

In a highly preferred variant, at least one further jet of cleaning gasis directed to the contamination layer for removing material from thecontamination layer. Using two or more jets of cleaning gassimultaneously allows one to speed up the cleaning, as the amount ofcleaning gas per time unit which can be produced in a cleaning head islimited due to technical constraints, leading to a cleaning rate oftypically about 0.05 nm/min when using atomic hydrogen as cleaningagent. Thus, in case that the exposure process has to be interrupted forthe cleaning process, the downtime of the EUV system required for thecleaning can be reduced. The use of more than one cleaning head istherefore particularly useful for EUV-reflective elements which arearranged at the beginning of the optical path, i.e. close to the EUVlight source, as these elements are exposed to EUV radiation with thehighest radiation intensity throughout the EUV system and thus thecontamination layers on their optical surfaces have a comparativelylarge thickness.

Moreover, the use of more than one jet of cleaning gas is advantageousas the uniformity of the cleaning process can be improved, since thejets of cleaning gas may be activated selectively, e.g. activating onlyone jet at a time, or activating several or all jets simultaneouslyduring different time intervals of the cleaning process. In particular,using two, preferably three or more jets of cleaning gas distributedaround the perimeter of the optical surface has been proven to be highlyadvantageous.

In a further preferred variant, the thickness of the contamination layeris monitored by generating a map of the thickness distribution of thecontamination layer. Such a map may be generated by directing monitoringlight to at least a sub-region, preferably to the entire optical surfaceand measuring the intensity of the light which is reflected from theoptical surface, respectively from the contamination layer, in aspatially resolving detector, the intensity of the reflected light beingindicative of the thickness of the contamination layer. Alternatively,the monitoring light may be directed to a single point on the opticalsurface, the location of which may be changed by displacing ortilting/rotating the source of monitoring light relative to the opticalsurface, such that the map can be generated in a scanning manner. In afurther alternative variant, in case that the cleaning head forgenerating the jet of cleaning gas is displaced laterally relative tothe optical surface and the jet of cleaning gas is essentially directedat a right angle to the optical surface, the monitoring light source andthe (not necessarily spatially resolving) detector may be displacedtogether with the cleaning head, e.g. by arranging them at appropriatelocations next to the cleaning head, thus measuring the thickness of thecontamination layer in the currently processed impact zone, which may besufficient when the cleaning is performed in a scanning manner.

In a highly preferred variant, the method comprises the further step ofgenerating the cleaning gas, the generation rate of the cleaning gasbeing controlled in dependence of the thickness of the contaminationlayer. In this way, the amount of cleaning gas per time unit may bereduced when the thickness of the contamination layer falls below acritical value. In the state of the art, as the actual thickness of thecontamination layer during the measurement is unknown, the generationrate of the cleaning gas is kept essentially constant during thecleaning process, i.e. normally the maximum possible generation rate isused.

In a further highly preferred variant, atomic hydrogen is used as acleaning gas when the thickness of the contamination layer falls below apre-determined thickness, preferably a thickness of 10 nm, morepreferably a thickness of 5 nm, in particular a thickness of 1 nm. Asatomic hydrogen cleaning is not a very aggressive cleaning method, moreaggressive cleaning methods may be used for speeding up the removal of,material from the contamination layer until the pre-determined thicknessis reached. Such aggressive and fast cleaning methods such as mechanicalcleaning using e.g. a blade or sputtering using a sputter gas of highatomic mass may be destructive to the topmost layer (cap layer) of themultilayer system even in case that a contamination layer with a smallthickness is still present on the optical surface. Therefore, it isadvisable to remove the remnant of the contamination layer using a lessaggressive cleaning method, e.g. by atomic hydrogen cleaning, which canbe very effectively used for this purpose. It is understood that, as thethickness of the contamination layer may vary over the optical surface,the choice of the cleaning gas or cleaning method can be made independence of the local thickness of the contamination layer.

In a highly preferred further development, before atomic hydrogen isused as a cleaning gas, material is removed from the contamination layerby another cleaning method, preferably selected from the groupconsisting of: sputtering, preferably using hydrogen, helium, argon,neon, or krypton as a sputter gas, mechanical cleaning by contactmethods, heating induced desorption, and chemical cleaning. Whensputtering is used as a cleaning method, a sputter gas is ionized andthe ions of the sputter gas are then accelerated in an electrical fieldand directed to the optical surface of the EUV-reflective element forremoving the contaminations. Using a sputter gas with a relatively lowatomic mass increases the selectivity for carbon contaminants. Thelarger the atomic mass of the sputter gas, the higher the probability toremove other materials from the optical surface as well, e.g. rutheniumfrom the cap layer, or silicon or molybdenum from the layers beneath thecap layer. Therefore, the sputter gas may be selected in dependence ofthe instantaneous thickness of the contamination layer, e.g. startingthe cleaning process with a sputter gas with a higher atomic mass andsubsequently switching over to a sputter gas with a smaller atomic masswhen the thickness of the contamination layer is reduced during thecleaning. Mechanical cleaning, e.g. using a scraper, chemical cleaning,e.g. by bringing a suitable cleaning agent (which dissolves carbon) intocontact with the optical surface, or heating-induced desorption, i.e.heating up the environment of the EUV reflective element or the opticalsurface of the EUV-reflective optical element itself eitherhomogeneously or locally, the latter e.g. by using a laser source as aheat source, directing radiation to a spot-like area on the opticalsurface. In general, in order to protect the optical surface, thesemethods should not be used when the thickness of the contamination layerfalls below a critical value.

In a further preferred variant, the cleaning is performed duringirradiation of the EUV-reflective optical element with EUV radiation. Inthis case, the entities which produce the jet(s) of cleaning gas as wellas the entities which are used for monitoring the thickness of theoptical surface are necessarily arranged outside of the optical path ofthe EUV light, thus allowing the cleaning process to be performed “inoperando”. It is understood that the cleaning process may also beperformed during the downtime of the EUV-lithography system. This may bethe case when the space in the EUV lithography system which is requiredfor arranging the above-described entities outside of the optical pathis insufficient, which may be the case for certain mirrors in the EUVlithography system.

In a highly preferred embodiment, the jet of cleaning gas is controlledfor removing the entire contamination layer from the optical surface. Inthis case, by using the feedback from the monitoring unit, evencontamination layers with a very irregular thickness distribution may beremoved from the optical surface without damaging the latter.

In a preferred alternative variant, the jet of cleaning gas iscontrolled for removing only part of the material of the contaminationlayer, the distribution of the material of the contamination layer afterthe cleaning having a desired shape. A locally varying thicknessdistribution of the contamination layer leads to a locally varyingreflectance of the EUV-reflective optical element. Therefore, by onlyselectively removing material from the contamination layer, e.g.removing material only in selected sub-regions of the optical surface, adesired optical property of the EUV-reflective optical element can begenerated, i.e. the desired shape of the contamination layer can bechosen such that system parameters of the EUV lithography system such astelecentricity, transmission, uniformity, ellipticentricity, etc. areoptimized. The desired shape of the contamination layer can bedetermined in a previous step by either using calculations ormeasurements of the optical properties of the EUV-lithography system.

In a preferred variant, the EUV-reflective element is arranged in anEUV-lithography system, and a signal indicative of at least one opticalcharacteristic of the EUV-lithography system is determined and is usedas an input signal for controlling of the jet of cleaning gas, theoptical characteristic being preferably chosen from the group consistingof: telecentricity, transmission, uniformity, ellipticity, and wavefronterror. In such a way, one or more signals generated by measurementequipment which is typically provided inside the EUV-lithography systemfor performing system measurements, e.g. measuring the intensitydistribution of the EUV radiation at the waver level using appropriatesensors (e.g. so-called slit sensors), may be used for determining thedesired shape of the contamination layer. Moreover, such a signal mayalso be used e.g. for determining when the initiation of a cleaningprocess of one or a plurality of EUV-reflective optical elements insideof the EUV-lithography system is required.

A further aspect of the invention is implemented in a method asdescribed in the introduction, the method comprising the step of:controlling the generation rate of the cleaning gas in a pulsed manner,the time duration of the cleaning gas pulses and the time durationbetween subsequent cleaning gas pulses being controlled such that thecleaning of the EUV-reflective optical element is optimized. Byselecting both the time duration of each cleaning gas pulse and the timeduration between the pulses in a specific way, the cleaning can beoptimized, as will be described in greater detail below.

In a highly preferred variant, the time duration of the cleaning gaspulses and the time duration between subsequent cleaning gas pulses arecontrolled such that a maximum temperature at or in the vicinity of theEUV-reflective element is not exceeded. When bringing a cleaning gasinto contact with the contamination layer, through the chemical reactionof the cleaning gas with the contamination layer, the optical surfacebeneath is heated up. As the materials which constitute the multilayersystem of the EUV-reflective element on which the optical surface isformed may be damaged due to excessive heating, it is proposed tocontrol the generation rate of the cleaning gas, i.e. generating lesscleaning gas in case that the temperature of the EUV-reflective elementor the temperature in the vicinity of the optical element attains agiven (maximum) value. In case that the instantaneous temperature of theEUV-reflective optical element needs to be determined, a temperaturesensor which generates a feedback signal for the control may be used.Alternatively, it is possible to determine the temperature of theEUV-reflective element by calculation of the thermal behaviour of theEUV-reflective system, taking into account the heating of the systemwhich is caused by the cleaning. In case that the cleaning is performedduring the exposure process, the heating caused by the exposure lighthas also to be taken into account for this calculation. The calculationmay further be supported by temperature measurements which have beenperformed beforehand. The time duration of the cleaning pulses and thetime duration between subsequent cleaning pulses which have to be usedto keep the temperature of the optical surface below the criticaltemperature depends on several parameters, e.g. the dimensions andweight of the specific EUV-reflective optical element as well as themaximum allowed temperature of the optical element including the heatingcaused by the exposure light, all of which may differ considerably amongthe optical elements of an EUV lithography system. The person skilled inthe art will appreciate that the time duration of the cleaning gaspulses and the time duration between subsequent cleaning gas pulses maybe modified during the cleaning for taking changes of the conditionsinside of the EUV lithography system into account.

In another highly preferred variant the time duration of the cleaninggas pulses and the time duration between subsequent cleaning gas pulsesare controlled such that generation of hydrogen-induced outgassingproducts is prevented. Hydrogen-induced outgassing (HIO) products aregenerated in a chemical reaction with components which are susceptibleof forming volatile compounds with atomic hydrogen. Such components aree.g. solder joints which comprise metallic substances such as tin orzinc, forming tin hydride and zinc hydride, respectively, when beingbrought into contact with atomic hydrogen. The problem with these andother hydrogen-induced outgassing products is that they may betransported to the optical surface of the EUV-reflective optical elementwhere the bare material, e.g. zinc or tin, is deposited as anirreversible contamination. Therefore, the production of suchhydrogen-induced outgassing products should be avoided. The approachwhich is described herein to avoid the production of these HIOs makesuse of the fact that in most cases, materials inside of the vacuumenvironment where the EUV-reflective optical element is arranged arepresent as oxides. In almost all cases, a jet of atomic hydrogen iscapable of reducing these oxides, such that the bare material mayevaporate or it may form a hydride with the atomic hydrogen, thelikelihood of either path being determined by the vapor pressure of thehydride and of the bare, typically metallic material. Ashydrogen-induced outgassing products typically generate irreversiblecontaminations on the optical surface of the EUV-reflective opticalelement (see above), in the presence of materials such as Sn, Zn, Mn,Na, P, S, Si etc. having a low vapor pressure, a pulsed cleaningstrategy is advantageous, as the process of reducing an oxide layer onsuch materials susceptible of producing HIO products using a jet ofatomic hydrogen takes a certain amount of time. When using pulsedcleaning, the time duration of the cleaning gas pulses may be controlledsuch that the oxide layers on those materials are not entirely removedand the time duration between the cleaning gas pulses may be chosen longenough such that the oxide layer can be restored. In such a way, it ispossible to perform hydrogen cleaning also in case that componentssusceptible of generating hydrogen-induced outgassing products arelocated at or in the vicinity of the EUV-reflective optical element.

In a highly preferred variant, the cleaning gas is generated byactivating a supply gas, the generation rate of the cleaning gas beingcontrolled by adjusting the activation rate of the supply gas,preferably in a pulsed manner. In this case, the cleaning gas isgenerated by activation of a supply gas, preferably molecular hydrogen,by bringing the supply gas in an excited (electron) state, or bydissociation, thus producing radicals or ions, as described e.g. in thepublication US 2003/0051739 A1, which is incorporated herein byreference in its entirety.

One example of such an activation process is the generation of atomichydrogen from molecular hydrogen using electrons as an activation means.The activation of the supply gas is preferably done in a pulsed manner,i.e. alternately using a first time period during which the activationis performed and a second time period during which no activation isperformed. For example, during a duty cycle, the activation may beperformed during a first time period, e.g. 10 min, after which theactivation may be stopped and the system is allowed to cool down againduring a second time period, e.g. 20 min. This duty cycle can beoptimized for a given layout of the EUV-lithography system. In the casedescribed above, the generation rate of the cleaning gas can becontrolled even though the supply gas is provided with constant pressureand flow rate.

In a preferred variant, a heated filament is used for activating thesupply gas, and for adjusting the activation rate, the temperature ofthe filament is controlled. The filament is heated for producingelectrons by thermo-emission. The electrons emitted from the filamentmay be accelerated in an electrical field and used to activate thesupply gas. Preferably, the filament is operated in a pulsed manner,i.e. switched on and held at a given temperature during a first timeperiod and switched off during a second time period of the duty cycle inwhich not only the heat produced by the cleaning process, but also theheat produced by the filament is no longer present in theEUV-lithography system. The given temperature may be set in dependenceof the environment of the EUV-reflective optical element inside theEUV-lithography system, i.e. it may vary in dependence of the positionof the EUV-reflective element. It is understood that the activation ratemay also be controlled by modifying the temperature of the filament in acontinuous instead of a pulsed manner.

In a highly preferred variant, the generation rate of the cleaning gasis controlled by adjusting the flow rate of the supply gas, preferablyin a pulsed manner. Thus, the flow rate of the supply gas can be usedfor influencing the generation rate of the cleaning gas. In this case,the filament or any other suitable device for activating the cleaninggas can be permanently switched on and still the generation rate can becontrolled. Providing the supply gas in a pulsed manner is particularlyadvantageous, as the EUV-lithography system is operated under vacuumconditions. Therefore, in those time periods during which no supply gasis provided, the non-activated part of the cleaning gas which is presentat the optical surface will be transported away, such that whenswitching on the supply gas again, the cleaning gas will be transportedfast because it is not hampered by diffusion in the background gas. Inthis case, the cleaning rate can be enhanced although the supply gas isswitched off for a certain amount of time during the duty cycle. Theperson skilled in the art will appreciate that also in this case, thetime duration of the cleaning gas pulses and the time duration betweensubsequent cleaning gas pulses may be controlled such that the transportof the cleaning gas to the contamination layer will not be limited bydiffusion.

It is to be understood that both principles, i.e. pulsed activation andpulsed gas flow operation can be advantageously combined, i.e. byswitching on the filament a few seconds before switching on the gasflow, and switching off the gas flow together with the filament.Moreover, both principles are also applicable to other cleaning gasproduction techniques, e.g. using RF. It is understood that thegeneration of the cleaning gas may be performed either off-line (i.e.during down time) or on-line (during normal operation of the EUVsystem), and that the pulsing strategy is useful in both cases. Inparticular, for on-line cleaning, pulsing may have the additionaladvantage that the average carbon cleaning rate and averagecarbonization rate (due to exposure to EUV light) can be balanced.

In a further preferred variant, the pressure of the supply gas is chosento be in a range from 10⁻³ mbar to 1 mbar, preferably in a range from0.05 mbar to 0.5 mbar, more preferably in a range from 0.1 mbar to 0.2mbar. Under these conditions, which are preferably used for thecleaning, the transport rate of the cleaning gas (and also of thenon-activated supply gas) to the surface of the EUV-reflective elementis limited by diffusion, such that, as described above, using a pulsedgas flow is particularly advantageous.

In a further highly preferred variant, the cleaning gas is directed as ajet of cleaning gas to the contamination layer in a method for removinga contamination layer from an optical surface of an EUV-reflectiveoptical element as described above, such that both methods may beadvantageously combined.

A further aspect is implemented in a cleaning arrangement for at leastpartially removing a contamination layer from an EUV-reflective opticalelement, the cleaning arrangement comprising: a cleaning head fordirecting a jet of cleaning gas to the contamination layer for removingmaterial from the contamination layer, a monitoring unit for monitoringthe contamination layer for generating a signal indicative of thethickness of the contamination layer, at least one movement mechanismfor moving the cleaning head relative to the optical surface, and acontrol unit for controlling the movement of the cleaning head using thesignal indicative of the thickness of the contamination layer as afeedback signal. In the cleaning arrangement, control of the cleaninghead is performed in dependence of the thickness of the contaminationlayer, thus allowing one to modify the cleaning process in dependence ofthe thickness of the contamination layer, in particular selectingdifferent cleaning methods or cleaning gases in dependence of thethickness of the contamination layer. Moreover, the location of theimpact zone of the jet of cleaning gas as well as its size and shape maybe controlled in dependence of the thickness of the contamination layer.In particular, by an appropriate movement of the cleaning head, thecleaning of areas on the optical surface which are free fromcontaminations can be avoided.

In a preferred embodiment, the cleaning arrangement comprises at leastone further cleaning head for directing a further jet of cleaning gas tothe contamination layer. Using more than one cleaning head, theuniformity of the cleaning can be improved and the cleaning process canbe speeded up, as the amount of cleaning gas per time unit which isavailable for the cleaning is increased. Moreover, the construction ofthe cleaning heads may be different, e.g. each cleaning head may beoptimized for the production of a specific cleaning gas, or thegeneration mechanisms for the production of the cleaning gas may varyamong the cleaning heads.

In a highly preferred embodiment, the monitoring unit comprises aspatially resolving detector for generating a map of the thickness ofthe contamination layer. By using a spatially resolving detector, thethickness distribution of the contamination layer can be determined,i.e. a three-dimensional map of the contamination layer can be generatedby measuring the spatially resolved reflectance of monitoring light fromthe optical surface.

In a further preferred embodiment, the monitoring unit comprises atleast one light source, electron source or ion source for directingmonitoring light, monitoring electrons or monitoring ions to the opticalsurface. The light source may be designed to produce monitoring light ina directed manner, which is the case, for example, with LEDs. In thiscase, the monitoring light can be directed to several points on theoptical surface with the same angle of incidence, e.g. by using a beamsplitter or optical guides, e.g. fibers etc. In case that a light sourceemitting light into different directions is used, the angle of incidencemay vary depending on the location on the optical surface. As therelationship between the thickness of the contamination layer and theintensity of the reflected light is dependent on the incidence angle, ineach monitored surface point, the incidence angle has to be taken intoaccount for producing the correct value of the thickness of thecontamination layer. For details of the monitoring process, reference ismade to German patent publication DE 10 2007 037 942.2 by the applicant,which is incorporated herein by reference in its entirety. Alternativelyor in addition, the monitoring unit may comprise an electron source oran ion source which may be used to perform electron spectroscopy or ionspectroscopy, respectively.

In another preferred embodiment, the movement mechanism comprises atleast one translatory drive for displacing the cleaning head in thedirection of at least one axis. By translation of the cleaning head inat least two directions, the position of the cleaning head and thus theimpact zone of the jet of cleaning gas on the contamination layer may bevaried in a scanning manner.

In a further preferred embodiment, the movement mechanism comprises atilting mechanism for tilting the cleaning head relative to the opticalsurface. The tilting mechanism allows changing the direction, i.e.rotating the jet of cleaning gas relative to the optical surface. When ajet of cleaning gas with a relatively small impact zone is used, also inthis case, the cleaning may be performed in a scanning manner. As lessspace is required inside the EUV system for tilting the cleaning headduring the cleaning process compared to displacing the cleaning headwith a translatory drive, this embodiment is especially suited forin-operando cleaning, as the cleaning head does not have to cross theoptical path for performing the cleaning.

In a further preferred embodiment, the control unit is designed tocontrol the movement of the cleaning head in a scanning manner, thusallowing cleaning of the optical surface in a systematic and moreuniform way.

A further aspect is implemented in a cleaning gas generationarrangement, comprising: a cleaning gas generator for generating a jetof cleaning gas to be directed to a contamination layer on an opticalsurface of an EUV-reflective optical element, and a control unit forcontrolling the generation rate of the cleaning gas in a pulsed manner,the time duration of the cleaning gas pulses and the time durationbetween subsequent cleaning gas pulses being controlled such that thecleaning of the EUV-reflective optical element is optimized.

In a preferred embodiment, the control unit controls the time durationof the cleaning gas pulses and the time duration between subsequentcleaning gas pulses such that a maximum temperature at or in thevicinity of the EUV-reflective element is not exceeded. In such a way,damage of the optical surface by heating up the optical surface due tothe reaction of the cleaning gas with the contamination layer can beavoided. Moreover, in such a way, it is possible to avoid imaging errorswhich may be caused by local expansion of the multilayer system and/orthe substrate of the EUV-reflective element. As discussed in greaterdetail above, the temperature may be determined by a calculation whichmay be based on data received in previous measurements. For a precisedetermination of the temperature during the cleaning process, thecleaning arrangement preferably further comprises a temperature sensorfor detecting the temperature at or in the vicinity of theEUV-reflective element.

Preferably, the control unit controls the time duration of the cleaninggas pulses and the time duration between subsequent cleaning gas pulsessuch that generation of hydrogen-induced outgassing products isprevented. As described in greater detail above, this can be done bychoosing the duration of the cleaning gas pulses sufficiently short forpreventing the removal of oxide layers on components which aresusceptible of outgassing hydrogen-induced outgassing products, and bychoosing the duration between the cleaning gas pulses sufficiently longfor regrowing of the oxide layers. Preferably, the growth of the oxidelayer is accelerated by an oxidizing agent such as oxygen gas or watervapor which is brought into contact with the component, in particular ina localized manner preferably in time slots when no illumination occurs(EUV light off conditions). In order to check that the cleaning iscorrectly performed, a gas detector, e.g. a mass spectrometer, may beprovided, the cleaning gas pulse being switched off immediately when thegas detector detects outgassing products. The time duration of thecleaning pulses and the time duration between subsequent cleaning pulseswhich have to be used to prevent generation of hydrogen-inducedoutgassing products may depend on several parameters, e.g. the type ofthe oxide, the thickness of the oxide layer etc., which may differconsiderably in dependence of the type and material of the componentwhich is susceptible of generating hydrogen-induced outgassing products.

In a highly preferred embodiment, the cleaning gas generator comprisesan activation unit for generation of the cleaning gas by activation of asupply gas, the control unit being designed to control the generationrate of the cleaning gas by adjusting the activation rate of the supplygas, preferably in a pulsed manner. Preferably, the activation unitcomprises at least one heated filament, the control unit being designedto control the temperature of the filament for adjusting the activationrate. For this purpose, a current through the filament can be controlledin such a way that a desired temperature and thus emission current isproduced. In addition, the difference between the potentials of thefilament and of a counter-electrode may be adjusted for influencing theacceleration of the electrons and thus the activation rate.

In a further preferred embodiment, the cleaning gas generationarrangement comprises a supply gas provision unit, the control unitbeing designed to control the flow rate of the supply gas which is fedfrom the supply gas provision unit to the activation unit, preferably ina pulsed manner. The flow rate of the supply gas may be set using avariable pump for pumping the supply gas from a gas reservoir to theactivation unit.

In yet another preferred embodiment, the supply gas provision unit isdesigned to provide the supply gas at a pressure in a range from 10⁻³mbar to 1 mbar, preferably in a range from 0.05 mbar to 0.5 mbar, morepreferably in a range from 0.1 mbar to 0.2 mbar. In a vacuum chamberwith a partial pressure of the supply gas, preferably hydrogen, in theabove range, the flow of the cleaning gas to the optical surfaces islimited by diffusion. Therefore, by providing the supply gas in a pulsedmanner, the cleaning process can be speeded up, as discussed in greaterdetail above.

A further aspect is implemented in a EUV-lithography system for imaginga structure on a photomask onto a light-sensitive substrate, comprising:at least one EUV-reflective optical element, at least one cleaningarrangement as described above and/or at least one cleaning gasgeneration arrangement as described above. It is to be understood thatthe cleaning arrangement and the cleaning gas generation arrangement canbe advantageously combined, the cleaning gas generation arrangementbeing used to generate the jet of cleaning gas which is then controlledby the cleaning arrangement. When combining the two arrangements, only asingle control unit may be used. Preferably, the cleaning/cleaning gasgeneration arrangements of different EUV-reflective elements are of adifferent construction, being adapted to the specific EUV-reflectiveelements for which they are employed.

In a highly preferred embodiment, the EUV-lithography system comprises aEUV light source, the EUV-reflective element being arranged in anoptical path of the EUV light source, the monitoring unit and/or thecleaning head being arranged outside of the optical path. In this way,in-situ cleaning of the EUV-reflective optical element can be performed.It is understood that when cleaning is performed during the downtime ofthe EUV-lithography system, one or more cleaning heads may also be movedinto the optical path for performing the cleaning. However, whenperforming in-operando cleaning, none of the components required for thecleaning may be moved into the optical path, as the exposure processshould not be adversely affected by the cleaning.

Further features and advantages are stated in the following descriptionof exemplary embodiments, with reference to the figures of the drawingwhich shows significant details, and are defined by the claims.Individual features can each be used singly, or several of them can betaken together in any desired combination, in order to implement desiredvariations.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described with reference to the drawingswherein:

FIG. 1 is a schematic of an EUV-lithography system according to anembodiment of the invention;

FIG. 2 is a schematic of an embodiment of a cleaning arrangement forcleaning one of the EUV-reflective elements of the EUV-lithographysystem of FIG. 1;

FIG. 3 is a schematic of a further embodiment of a cleaning arrangementfor cleaning a further EUV-reflective element of the EUV-lithographysystem of FIG. 1;

FIG. 4 is a schematic of an embodiment of a cleaning gas generationarrangement for the generation of atomic hydrogen;

FIG. 5 is a diagrammatic view of the duty cycles of an activation unitand of a supply gas provision unit of the cleaning gas generationarrangement of FIG. 4;

FIG. 6 is a schematic of a further embodiment of a cleaning gasgeneration arrangement; and,

FIG. 7 is a diagrammatic view of the duty cycles of the cleaning gasgeneration arrangement of FIG. 6.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 shows a schematic representation of a EUV-lithography systemwhich is designed for manufacturing highly integrated semiconductordevices. The EUV-lithography system 1 comprises a beam shaping system 2,an illumination system 3 and a projection system 4, each of which isarranged in a separate vacuum compartment. The beam shaping system 2comprises a EUV light source 5 which may be implemented as a plasmasource or as a synchrotron source and emits EUV light which forms anoptical path 6 through the EUV-lithography system 1. The EUV lightemitted in a wavelength range of between 5 nm and 20 nm from the EUVlight source 5 is first fed to a collimator 7 and subsequently, thedesired operating wavelength for the exposure process (typically 13.5nm) is selected by adjusting the incidence angle (see the double-headedarrow) of the EUV light which impinges on an optical surface 8 a of amonochromator 8. The collimator 7 and the monochromator 8 are in generalimplemented as reflective optical elements.

The illumination system 3 serves to generate a homogeneous radiationdistribution from the EUV-light which is provided by the beam shapingsystem 2 and comprises a first and second EUV-reflective optical element(9, 10), each having an optical surface (9 a, 10 a) positioned forreflecting the EUV light to a photomask 11 as a further EUV-reflectiveoptical element which comprises a pattern which is imaged in a reducedscale onto a light-sensitive substrate of a wafer 12 by the projectionsystem 4. For this purpose, the projection system 4 comprises a thirdand fourth EUV-reflective optical element (13, 14), each having arespective optical surface (13 a, 14 a) for directing the EUV light ontothe wafer, the wafer 12 being arranged in an image plane of theprojection system 4.

The EUV-reflective elements 8 to 11, 13 and 14 are subject tocontamination due to impurities, e.g. by hydrocarbon molecules, whichcannot be avoided even though the compartments 2 to 4 are operated undervacuum conditions. These hydrocarbon molecules react with the EUV lightin the optical path 6 such that carbon deposits are generated on theoptical surfaces 8 a to 10 a, 13 a, 14 a of the correspondingEUV-reflective elements 8 to 10, 13, 14. In the following, suitablearrangements will be described for removing the contamination from thesesurfaces, the arrangements not being represented in FIG. 1 for the sakeof simplicity.

FIG. 2 shows that carbon deposits on the optical surface 14 a of thefourth optical element 14 of the EUV-lithography system 1 of FIG. 1 forma contamination layer 15 on the optical surface 14 a. The opticalsurface 14 a is located on the topmost layer (cap layer) 17 of amultilayer system 16 having alternating molybdenum and silicon layers.The multilayer system 16 is arranged on a substrate 18 of theEUV-reflective element 14. For at least partially removing thecontamination layer 15 from the optical surface 14 a, a cleaningarrangement is provided in the EUV-lithography system 1. The cleaningarrangement comprises a cleaning head 19 which directs a jet 20 ofatomic hydrogen as a cleaning gas to the contamination layer 15 forremoving material from the optical surface 14 a as shown in the upperleft corner of FIG. 2.

The cleaning head 19 is mounted on a translatory drive 21 serving as amovement mechanism for displacing the cleaning head 19 relative to theoptical surface 14 a in a plane which is defined by first and secondaxes (X, Y). The movement of the translatory drive 21 and thus of thecleaning head 19 is controlled by a control unit 22 which is designedfor moving the cleaning head 19 in a scanning manner over the opticalsurface 14 a.

As shown in the lower left corner of FIG. 2, the contamination layer 15is discontinuous and forms only small contaminating spots on the opticalsurface 14 a, such that the jet 20 of cleaning gas has to be directedonly to these spots for the removal of the contamination layer 15 inorder to avoid damaging of the optical surface 14 a by the cleaning gas.As a precise positioning of the cleaning head 19 is required in thiscase, the latter has a small spot size, i.e. a small impact zone on theoptical surface 14 a. It is understood that the size of the impact zonemay be adjusted by moving the cleaning head 19 also along a third axis Zperpendicular to the optical surface. As the cleaning head 19 isdisplaced over the optical surface 14 a in a scanning manner, thecleaning head 19 is necessarily brought into the optical path 6 shown inFIG. 1. Therefore, the exposure process has to be interrupted during thecleaning process.

In order to determine the correct locations on the optical surface 14 ato which the jet 20 of cleaning gas has to be directed, the cleaningarrangement comprises a monitoring unit for inspecting the opticalsurface 14 a and the contamination layer 15, respectively, shown in thelower right corner of FIG. 2. The monitoring unit comprises a monitoringlight source 23 and a spatially resolving detector 24, both of which arearranged outside of the optical path 6 (shown in FIG. 1) so as not todisturb the exposure process. Monitoring light 25 is emitted by themonitoring light source 22 over a large solid angle covering the entireoptical surface 14 a and the reflected monitoring light 25 is detectedin the spatially resolving detector 24. The intensity of the monitoringlight reflected from the optical surface 14 a is indicative of thethickness of the contamination layer 15. The thicker the contaminationlayer 15, the smaller the amount of monitoring light reflected from theoptical surface 14 a. The intensity signal produced by the detector 24is fed to the control unit 22 operatively coupled to the detector 24.The control unit 22 generates a three-dimensional map 26 of the opticalsurface, a two-dimensional sectional view of which is shown in the upperright corner of FIG. 2. By generating a three-dimensional map, it is notonly possible to determine the location of the contaminating spots onthe optical surface 14 a, but also the thickness of the contaminationlayer 15 in these spots. This is possible as the control unit 22 isdesigned to calculate a thickness distribution from the spatiallyresolved intensity distribution of the detector 24 by using a known(e.g. determined by prior measurements) correlation between theintensity of the reflected monitoring light 25 and the thickness of thecontamination layer 15. In particular, for generating thethree-dimensional map 26, the dependence of the correlation on the angleof incidence of the monitoring light 25 on the optical surface 14 a hasto be taken into account, as described in detail in DE 10 2007 037 942.2by the applicant. In particular; in order to increase the sensitivity ofthe measurement, it is advisable to choose a wavelength of themonitoring light 25 in which the change of the intensity signal within agiven thickness range of the contamination layer 15 can be maximized fora given angle of incidence, which is possible e.g. when choosing awavelength in the visible domain, i.e. in a wavelength range from 400 nmto 800 nm.

The information about the thickness distribution of the contaminationlayer 15 on the optical surface 14 a can not only be used to direct thecleaning head 19 to the correct locations on the optical surface 14 a,but also to adjust the cleaning time, i.e., the time in which the jet 20of cleaning gas has to be directed to a particular location on theoptical surface 14 a for entirely removing the contamination layer 15through online monitoring of the reduction of the thickness of thecontamination layer 15.

The person skilled in the art will appreciate that instead of usingatomic hydrogen as a cleaning gas, also other suitable cleaning gasesmay be employed, such as atomic nitrogen, halogenides (Br, I, etc.),oxygen radicals, argon radicals, hydrogen radicals, neon radicals,helium radicals, krypton radicals, pure gas plasmas and their mixtures,in particular argon plasma and oxygen plasma. The construction of thecleaning head 19 may be adapted to the cleaning gas used, or may beadapted for the use of more than one cleaning gas which are thenprovided to the cleaning head in turn. In particular, the choice of thecleaning gas or of the cleaning method may vary depending on thethickness of the contamination layer, as will be described in thefollowing with respect to FIG. 3.

FIG. 3 shows the first EUV-reflective optical element 9 of FIG. 1together with a further cleaning arrangement being used to remove acontamination layer 15′ from its optical surface 9 a. The cleaningarrangement of FIG. 3 comprises two cleaning heads (19 a, 19 b)producing respective jets (20 a, 20 b) of cleaning gas. The cleaningheads (19 a, 19 b) are connected to respective tilting mechanisms (27 a,27 b) for changing the direction of the jets (20 a, 20 b) of cleaninggas relative to the optical surface 9 a. The tilting mechanism, e.g. aswivelable arm, is constructed in such a way that the jets (20 a, 20 b)of cleaning gas can reach any point on the optical surface 9 a forperforming the cleaning. In the example of FIG. 3, the contaminationlayer 15′ is a continuous, relatively thick layer, having a thickness ofabout 15 nm in a surface area on the left-hand side, and a reducedthickness of less than 5 nm in a surface area close to the right-handside. The first of the two cleaning heads 19 a uses helium as a sputtergas for the cleaning, being more aggressive than atomic hydrogen whichis used as cleaning gas in the second jet 20 b of cleaning gas. In orderto avoid damaging the optical surface, the first cleaning head 19 a isused to reduce the thickness of the contamination layer 15′ only up toan thickness of 5 nm, whereas the second cleaning head 19 b is used toremove the remnant of the contamination layer 15′. For the sputtering, apotential difference between the EUV-reflective optical element 9 andthe cleaning head 19 a is generated by a voltage generator (not shown)for accelerating the helium ions in the jet 20 a of cleaning gasdirected to the optical surface 9 a. It is understood that other sputtergases, e.g. hydrogen, helium, argon, neon, or krypton may be used aswell. Alternatively or in addition, it is possible to remove thecontamination layer up to a certain thickness, e.g. 5 nm, by othercleaning methods, including mechanical cleaning by contact methods,heating induced desorption, or chemical cleaning.

Of course, it is also possible for both cleaning heads (19 a, 19 b) touse the same cleaning gas for increasing the uniformity of the cleaning.It is understood that more than two cleaning heads may also be used forincreasing uniformity, being preferably arranged equally spaced in acircumferential direction around the perimeter of the EUV-reflectiveoptical element 9.

The cleaning arrangement shown in FIG. 3 differs from the one shown inFIG. 2 not only in the number of cleaning heads, but also in the way thethree-dimensional map 26 of the contamination layer 15′ is generated, asectional view of which is shown in the upper right corner of FIG. 3. Inthe monitoring unit of FIG. 3, a laser diode is used as a light source23′ which emits monitoring light 25′ in the visible wavelength region ina directed manner to the optical surface 9 a, as shown in the lowerright-hand corner. The monitoring light 25′ from the light source 23′impinges on a beam splitter 23 a which transmits a first portion of themonitoring light 25′ to a first point on the optical surface 9 a andreflects a second portion of the monitoring light 25′ to a mirror 23 bwhich reflects the second portion of the monitoring light 25′ to asecond point on the optical surface 9 a. In this way, the monitoringlight 25′ impinges on the two points on the optical surface 9 a at thesame angle of incidence, such that no correction for the incidence angleis necessary for calculating the thickness from the intensity signalwhich is detected by a spatially resolving detector 24′. The personskilled in the art will appreciate that the number of points that themonitoring light 25′ impinges on can be adjusted by using additionalbeam splitters or by using more than one monitoring light source. It isunderstood that reflectometry may be performed using light in the UV orEUV wavelength range.

The person skilled in the art will appreciate that the thickness of thecontamination layer 15′ may be monitored using a monitoring unit whichmay be designed in a variety of ways. It is e.g. possible to use ahigh-resolution camera with imaging optics for inspecting the opticalsurface 9 a, the focus of the imaging optics being shifted for producingimages of sectional views of the contamination layer 15′ with varyingdistance from the optical surface 9 a. Also, monitoring the thickness ofthe contamination layer 15′ may be performed by measuring thephotocurrent induced by EUV or UV radiation impinging on the opticalsurface 9 a, or by classical surface analysis methods such as X-rayPhotoelectron Spectroscopy (XPS), Scanning Electron Microscopy (SEM),and Auger Electron Spectroscopy (AES).

In the example of the cleaning arrangement shown in FIG. 3, the cleaningcan be performed in-operando, as it is not necessary for the cleaningheads (19 a, 19 b) to be moved into the optical path 6 for removing thecontamination layer 15′. As the first, EUV-reflective element 9 isexposed to EUV light with the highest intensity of all theEUV-reflective elements 9 to 11, 13, 14, the contamination layer 15′grows at an elevated rate, such that in-operando cleaning isparticularly advantageous in this case to reduce downtimes of theEUV-lithography system 1.

It is understood that in addition to the tilting mechanism (27 a, 27 b),it is possible to use a movement mechanism which allows for displacingthe cleaning heads (19 a, 19 b) in a translatory motion, and that thecleaning head 19 of FIG. 2 may also be provided with an additionaltilting mechanism (not shown).

In the following, the generation of the jet 20 of cleaning gas shown inFIG. 2 is described with reference to FIG. 4, in which the cleaning head19 is shown in greater detail. For generating the jet 20 of atomichydrogen H•, the cleaning head 19 is connected to a hydrogen reservoir29, e.g. a bottle or another suitable vessel for pressurized hydrogen,which may be arranged at a remote location inside or outside of theEUV-lithography system 1. The molecular hydrogen H₂ is fed from thehydrogen reservoir 29 via a supply gas provision unit 30 using a pump(not shown) at a constant pressure of about 0.1 mbar to the interior ofthe cleaning head 19, which comprises a filament 31 and a voltagegenerator 34 for generating an electrical field 33 between the filament31 and a counter-electrode 34 a to which the voltage generator 31 isoperatively connected. The two ends of the filament 31 are connected toa power source 32 for heating the filament by passing an electricalcurrent through the filament 31. The filament 31 together with the powersupply 32, the voltage generator 31 and the counter-electrode 34 a areused as an activation unit for the molecular hydrogen H₂, as theyprovide accelerated electrons which are used to crack the molecularhydrogen H₂ to atomic hydrogen H•, which is then used as a cleaning gasin the jet 20. It is understood that normally, not the entire supply gascan be activated in the way described above, such that apart from thecleaning gas, also part of the supply gas is provided to the jet 20.

In order to prevent excessive heating of the optical element 14, thecontrol unit 22 is operatively connected to a temperature sensor 28which measures the temperature T in the vicinity of the optical surface14 a. The control unit 22 is designed to control both the supply gasprovision unit 30 (i.e. its pump power) for adjusting the flow rate ofthe molecular hydrogen H₂ and the power supply 32 for adjusting thetemperature of the filament 31 by controlling the current through thefilament 31 in dependence of the temperature T. Alternatively, thetemperature close to the optical surface 14 a may be determined bycalculations which simulate the heat transfer inside of theEUV-lithography system 1, taking the heat which is generated by thecleaning into account.

The control unit 22 is used to maintain the temperature T near theoptical surface 14 a below a critical temperature T_(MAX) which is about60° C. for standard multilayer mirrors and about 200° C. forhigh-temperature multilayer mirrors, the latter usually comprisingbarrier layers for preventing interdiffusion between the individuallayers, thus preventing the material of the multilayer system 16 fromdamage caused by the interdiffusion due to excessive heating. For thepurpose of not exceeding the critical temperature T_(MAX), the controlunit 22 controls both the temperature of the filament 31 and the flowrate of the supply gas in a pulsed manner, using a duty cycle as shownin FIG. 5.

During the duty cycle, first the filament 31 is heated for severalseconds by setting a current I through the filament 31 to a constantvalue. In this way, pre-heating of the filament 31 is achieved, so thatthe filament can be heated to a desired temperature before the gas flowis switched on and set to a flow rate F which is also constant. After acleaning pulse C1 of a time duration t1 of about two minutes, both thecurrent I and the flow rate F are set to zero by the control unit 22 fora time Δt1 of about four minutes. During this time, the vacuum pump (notshown), which produces a vacuum inside of the EUV-lithography system 1,pumps away the re-combined or non-activated cleaning gas as well as themixture of gases produced by the reaction of the atomic hydrogen withthe carbon from the contamination layer 15. Therefore, when the filament31 and the flow of cleaning gas are switched on once again, producing afurther cleaning pulse C2 when starting a subsequent duty cycle, theflow of the cleaning gas to the optical surface 14 a is fast, as it isnot hampered by diffusion in the background hydrogen gas present at theoptical surface 14 a during the cleaning, which has a partial pressureof about 0.1 mbar (corresponding essentially to the pressure of thehydrogen gas H₂ from the gas provision unit 30) and therefore limits thetransport rate of the atomic hydrogen H• to the optical surface 14 a.Therefore, by pulsed production of the cleaning gas, it is possible tospeed up the cleaning process as well as to prevent overheating of theoptical surface 14 a. The person skilled in the art will appreciate thatthe time durations (t1, Δt1) given above are only exemplary values andmay modify these values to make them fit to the conditions of eachindividual EUV reflective element within the EUV lithography systemwhich is to be cleaned.

It is understood that by increasing the hydrogen flow over the hotfilament and/or heating the filament to higher temperatures, a greaterfraction of the supply gas H₂ is activated and that consequently, thecleaning can be speeded up as well, which may be particularly usefulwhen the exposure process is interrupted during the cleaning. However,as e.g. heating the filament to higher temperatures also leads to anincreased heating of the environment, a compromise must be made betweenthe cleaning speed and the heating. For EUV-reflective elements whichare efficiently cooled, e.g. by using water as a cooling fluid, thefilament may be heated to higher temperatures, which is especiallydesirable when cleaning is performed on EUV-reflective optical elementssuch as the first or second EUV-reflective optical element (9, 10) ofthe illumination system 3 which in general exhibit a contamination layerhaving a relatively large thickness. For the third and fourthEUV-reflective optical elements (13, 14) of the projection system 4,heating is more critical and less contamination is expected, such thatthe temperature of the filament may be set to a lower value in thiscase. In any case, when performing pulsed cleaning, during the exposureprocess, the contamination growth and the removal rate of thecontaminations can be balanced, such that the contamination level of theEUV-reflective optical elements can be kept constant during theexposure.

FIG. 6 shows an arrangement which differs from the one shown in FIG. 4in that the cleaning head 19 does not comprise an entity which generatesan electrical field. The supply gas is passed over the filament 31 beingheated to temperatures of approximately 2000° C. which are sufficient tocrack the molecular hydrogen H₂ to form atomic hydrogen H•. For thepurpose of heating, the power source 32 it connected to both ends (35 a,35 b) of the filament 31. In the arrangement shown in FIG. 6, heating isless critical, as a cooling unit (not shown) is used for cooling theEUV-reflective optical element 14, such that the current I through thefilament 31 is kept constant at a temperature of about 2000° C. duringthe cleaning, as shown in FIG. 7 which also represents the flow rate Fof the cleaning gas.

The arrangement of FIG. 6 further differs from the arrangement of FIG. 4in that a component 35, which is susceptible of generatinghydrogen-induced outgassing products in the form of a solder comprisingzinc (Zn) or tin (Sn), is arranged on the substrate 18 of theEUV-reflective optical element 14. The cleaning head 19 is arranged at adistance of e.g. about 100 mm from the substrate 18 and as the component35 is located close to the optical surface 14 a to be cleaned, part ofthe cleaning gas from the jet 20 of cleaning gas is brought into contactwith the component 35 and reduces an oxide layer (not shown) on top ofthe component 35. As, after the removal of the entire oxide layer, thebare material may evaporate, or it may form a hydride with the atomichydrogen (forming a hydrogen-induced outgassing product) which typicallygenerates irreversible contaminations when deposited on the opticalsurface 14 a of the EUV-reflective optical element 14, in the presenceof materials such as Sn, Zn, Mn, Na, P, S, Si etc. having a low vaporpressure close to the optical surface 14 a, a pulsed cleaning strategyis advantageous, as the process of reducing the oxide layer on e.g. thecomponent 35 takes a certain amount of time. In the pulsed cleaning, thetime duration t2 (a few minutes, e.g. ten minutes) of the cleaning gaspulse C1 is controlled such that the oxide layer on the component 35 isnot entirely removed and the time duration Δt2 (e.g. about 20 minutes)between the cleaning gas pulse C1 and the subsequent cleaning gas pulseC2 is selected long enough such that the oxide layer on the component 35can be restored, a process which may be accelerated by locallyintroducing an oxidizing agent close to the component 35. By usingpulsed cleaning in the way described above, it is possible to performhydrogen cleaning also in case that components susceptible of generatinghydrogen-induced outgassing products are located at or in the vicinityof the EUV-reflective optical element. To make certain that nohydrogen-induced outgassing products are generated during the cleaningprocess, a gas detector 36 in form of a mass spectrometer is connectedto the control unit 22′, such that the cleaning gas jet 20 can beswitched off immediately in case that hydrogen-induced outgassingproducts such as tin, zinc or their hydrides are detected. In this way,the initiation of the restoration of the oxide layer can be triggered bythe detection of outgassing products.

The person skilled in the art will appreciate that instead of usingcleaning gas pulses (C1, C2) with a rectangular shape as shown in FIG. 5and FIG. 7, cleaning gas pulses of a different pulse shape may also beused for the cleaning, the time duration of these pulses being definedas usual in this case as full width at half maximum.

By the arrangements and methods described above, the contaminationlayers (15, 15′) may be entirely removed from the optical surfaces (9 a,14 a) in a very efficient way without damaging the material of themultilayer system 16. However, instead of entirely removing thecontamination layer 15′ from the optical surface 14 a, it is possible toremove only part of this layer, i.e. to generate a contamination layerof a desired shape through the cleaning, the shape of the contaminationlayer being adapted for the generation of a desired optical property ofthe EUV-lithography system 1, e.g. by optimizing its telecentricity. Thecontrol unit 22′ or a further calculation unit (not shown) may bedesigned to calculate the desired shape of the contamination layers onthe EUV-reflective optical elements 9 to 11, 13, 14 based onmeasurements of the optical properties of the EUV-lithography system 1which may be performed e.g. by measuring the intensity distribution ofthe EUV radiation at the wafer level using appropriate sensors (e.g.so-called slit sensors). However, for the discontinuous contaminationlayer 15 shown in FIG. 2, there is not enough material to generate of adesired shape, such that the contamination layer 15 is entirely removedfrom the optical surface 14 a.

Apart from carbon, other contaminating substances may be deposited onEUV-reflective optical elements as well, such as contaminants outgassingfrom a photoresist on the wafer 12, e.g. sulphur compounds or metalhydrides released from the EUV source or from special materials duringcleaning. The person skilled in the art will appreciate that suchcontaminants may also be efficiently removed or avoided by using themethods and arrangements as described above.

The above description of the preferred embodiments has been given by wayof example. From the disclosure given, those skilled in the art will notonly understand the present invention and its attendant advantages, butwill also find apparent various changes and modifications to thestructures and methods disclosed. The applicant seeks, therefore, tocover all such changes and modifications as fall within the spirit andscope of the invention, as defined by the appended claims, andequivalents thereof.

It is understood that the foregoing description is that of the preferredembodiments of the invention and that various changes and modificationsmay be made thereto without departing from the spirit and scope of theinvention as defined in the appended claims.

1. A cleaning gas generation arrangement for cleaning an EUV-reflectiveoptical element defining an optical surface on which a contaminationlayer can form, the arrangement comprising: a cleaning gas generator forgenerating a jet of cleaning gas directed to the contamination layer onsaid optical surface of said optical element; a control unit forcontrolling the generation of said cleaning gas as a sequence of pulsesthereof; and, said control unit being adapted to control the timeduration of said pulses and the time duration between said pulses so asto optimize the cleaning of said optical element.
 2. The cleaning gasgeneration arrangement of claim 1, wherein said control unit is adaptedto control the time duration of said pulses and the time durationbetween said pulses such that a maximum temperature (T_(MAX)) at or inthe vicinity of said optical element is not exceeded.
 3. The cleaninggas generation arrangement of claim 1, wherein the control unit isadapted to control said time duration of said pulses and the timeduration between said pulses so as to prevent a generation ofhydrogen-induced outgassing products.
 4. The cleaning gas generationarrangement of claim 3, further comprising a gas detector for detectinghydrogen-induced outgassing products.
 5. The cleaning gas generationarrangement of claim 1, wherein said arrangement further comprises asource of supply gas; said cleaning gas generator further comprises anactivation unit for generating said cleaning gas by activating saidsupply gas; and, said control unit is adapted to control the rate ofgeneration of said cleaning gas by adjusting the rate of activation ofsaid supply gas.
 6. A cleaning arrangement for at least partiallyremoving a contamination layer from an EUV-reflective optical elementdefining an optical surface, the cleaning arrangement comprising: acleaning head for directing a jet of cleaning gas to the contaminationlayer for removing material from the contamination layer; a monitoringunit for monitoring the contamination layer and for generating a signalindicative of the thickness of the contamination layer; a movementmechanism for moving said cleaning head relative to said opticalsurface; and, a control unit for controlling the movement of saidcleaning head using said signal indicative of the thickness of thecontamination layer as a feedback signal.
 7. The cleaning arrangement ofclaim 6, wherein the movement mechanism comprises at least one of atranslatory drive for displacing said cleaning head in the direction ofat least one axis (x, y, z) and a tilting mechanism for rotating saidcleaning head relative to said optical surface.
 8. The cleaningarrangement of claim 7, wherein said control unit is designed to controlthe movement of said cleaning head in a scanning manner.
 9. A method forat least partially removing a contamination layer from an opticalsurface of an EUV-reflective optical element, the method comprising thesteps of: directing a jet of a cleaning gas to contact the contaminationlayer for removing material from the contamination layer; monitoring thecontamination layer for generating a signal indicative of the thicknessof the contamination layer; and, moving the jet of cleaning gas relativeto the optical surface in a controlled manner by using said signal as afeedback signal.
 10. The method of claim 9, wherein the movement of thejet of cleaning gas is controlled by displacing the jet of cleaning gasor by changing the direction of the jet of cleaning gas relative to saidoptical surface.
 11. The method of claim 9, wherein said jet of cleaninggas is displaced or tilted relative to said optical surface in ascanning manner.
 12. The method of claim 9, wherein atomic hydrogen (H•)is used as said cleaning gas when the thickness of the contaminationlayer falls below a predetermined thickness; and, before atomic hydrogen(H•) is used as a cleaning gas, material is removed from thecontamination layer by another cleaning method.
 13. The method of claim9, wherein said cleaning gas is selected from the group consisting of:atomic hydrogen, atomic nitrogen, halogenides, oxygen radicals, argonradicals, hydrogen radicals, neon radicals, helium radicals, kryptonradicals, pure gas plasmas and their mixtures including argon plasma andoxygen plasma.
 14. The method of claim 9, wherein said jet of cleaninggas is controlled for removing only part of the material of thecontamination layer so as to leave a distribution of the material of thecontamination layer on said optical surface to have a predeterminedshape.
 15. The method of claim 9, wherein said optical element isarranged in an EUV-lithography system and a signal indicative of atleast one optical characteristic of the EUV-lithography system isdetermined and applied as an input signal for controlling said jet ofsaid cleaning gas.
 16. A method for generating a jet of cleaning gas tobe directed to a contamination layer on an optical surface of anEUV-reflective optical element, the method comprising the step of:controlling the rate of generation of said cleaning gas as a sequence ofpulses thereof by controlling the time duration of said pulses and thetime duration between said pulses so as to optimize the cleaning of saidoptical element.
 17. The method of claim 16, wherein the time durationof said cleaning gas pulses and the time duration between said cleaninggas pulses is controlled such that a maximum temperature (T_(MAX)) at orin the vicinity of said optical element is not exceeded.
 18. The methodof claim 16, wherein the time duration of said cleaning gas pulses andthe time duration between said cleaning gas pulses is controlled so asto prevent a generation of hydrogen-induced outgassing products.
 19. Themethod of claim 16, comprising the further steps of: providing a supplygas; generating said cleaning gas by activating said supply gas; and,controlling the rate of generation of said cleaning gas by adjusting therate of activation of said supply gas.
 20. The method of claim 16,comprising the further steps of: providing a supply gas; applying aheated filament to activate said supply gas; and, controlling thetemperature of said filament for adjusting the rate of activation ofsaid supply gas.